....and Gore-Tex ( Goretex) bivy covers fro USM sleep systems
So great is the taste for these in new and fully waterproof condition, those repaired ones are available for a song in the surplus market. 68" circumference as shoulder and hips, tapering to 38" at the foot.
About 80" long, with all the physical properties of expensive Gore-Tex, a draft flap with snaps, and a fine double YKK zipper to boot.
Get a partner / helper to pin one to a custom taper with you and UQ in the hammock the bivy sack is to be fit to. Get out, and trim to reduce fabric and weight, if desired.